1. Field of the Invention
The present invention relates to a temperature dependent current sense circuit for a substantially constant intensity light source. Specifically, the present invention relates to a current sense circuit for a LED matrix for use in signal lights such as traffic lights.
2. Description of the Prior Art
The light-emitting diode (LED) produces light when a forward current flows thru its P-N junction. The intensity of the emitted light is directly proportional to the forward current, IF, and is maintained constant by regulating IF. Changing If according to the temperature profile of the LED performs thermal compensation of the light intensity.
The light-emitting diode (LED) can be driven by a constant voltage, VF, to get the desired forward current, IF, or to regulate directly IF. The non-linear electrical characteristic of IF/VF has a logarithmic profile. Regulating IF by using the voltage driven method will result to non-linear variations of IF for any variations of VF, such as initial, temperature and aging variations. Regulating directly IF overcomes the non-linearity mentioned above and this is the selected method used in the prior art. Thermal compensation is performed at the level of RS, see FIG. 2, i.e. the equivalent resistance value RS is the result of the combination of a thermistor in parallel with a resistor and in series with another resistor. The selection of these components' value determines the required thermal profile of the LED lamp.
As shown in FIG. 2, the conventional LED current sense circuit has been used for 5 mm LED lamps. The LED array (not shown in FIG. 2) is fed via J1 and the lamp current returns back to the power transformer through the current sense resistor, RS. The controller CTL drives Q1 in such a way that the voltage at pin 1 of CTL is maintained at 2.5V (+2.5V internal reference). In steady state, the voltage across RS is 2.5 Vdc and the current flowing through it is determined by its resistance value. In the example of FIG. 2, the resistance value of RS is 25 ohms and the lamp current is regulated to 100 mA. The power dissipation in the sense resistor RS, as shown in FIG. 3, is 0.25 W (100 mA through 25 ohms).
For a 1 W LED applications, the load current that must be sensed is in the order of 1A and the power dissipation of RS would be 2.5 W thus reducing the efficiency of the power supply. One way to reduce the power loss is to reduce the current sense resistance and to amplify the sensed voltage, as shown in FIG. 3. Using the resistor value of R1 and R2, for 1A load, the Op Amp gain is 38.5 (Gain=2.5V/0.065V or Gain=1+R1/R2) and the power dissipation is only 65 mW (Pdiss=0.065*12).
In addition to the constant light intensity requirement of Traffic Light units, power factor control (PFC) must be incorporated. Insertion of the integrated power factor controller circuit in an electrical power supply system enables easy and efficient control of the power factor and level of current harmonics. To obtain a power factor equal to unity, prior art systems use controllers such as controller MC33262 from Motorola. The controller draws current from the ac source in proportion to the sinusoidal voltage. This automatically causes the current waveform to be sinusoidal and in phase with the line voltage waveform. But a PFC circuit has the drawback of having voltage ripple across the output filter capacitor at twice the line frequency. A certain amount of that voltage ripple appears across the current sense resistor and pin 1 of CTL. In the case that the voltage at pin 1 of CTL exceeds 2.7 volt, CTL stops driving Q1 and distortion of the line current waveforms occurs thus increasing the ATHD (Line current Total Harmonic Distortion). The selection of the output filter capacitor value of C7 is thus very critical. Any reduction of the capacitor value due to the initial tolerance and temperature can lead to the scenario mentioned above under some load conditions. An extra pole (R3, C3) is also required to attenuate the 120 Hz ripple to maintain the peak voltage of CTL-1 below 2.7V. Moreover, in order to accommodate different loads, the resistance of R3 must be selected which is not a good practice for a high volume production. Further, the additional pole adds more phase shift at the crossover frequency of the feedback loop and may lead to instability.
A fixed LED output current presents the following drawbacks: at higher temperature the output LED light intensity decreases; at lower temperature the output LED light intensity increases. One object of the present invention is to thermally regulate the output current, and thus the light intensity, of a non-linear high-power light-emitting load.
Secondly, prior art is very sensitive to the output capacitor variations that can lead to increase the ATHD and another object is to eliminate the concern associated to these variations.
As more powerful LEDs are used in Traffic Light units, there is a need for a stable monitoring circuit. One object of the present invention is an improved circuit.